Baycom modem

The PTT works but no signal (from the output or
the chip) I have gone through all the papers and discovered theres been a
problem before: mainly the "World Chip" (i think 7910?) has been
replaced once for unknown reasons. ??? i think it is a BP-1 (or other very
early model (there are no labels but it sounds like a BP-1 from descriptions))

If it is the chip where can you get replacements?

Any help (and all the suggestions) has made me
know their are still amatures who are interested :-)